1. Field
The present disclosure relates to a bidirectional DC/DC converter that carries out DC/DC conversion in two directions via an isolating transformer, and in particular, relates to a bidirectional DC/DC converter suited to an application wherein the input/output voltage range is wide, for example, as a battery charger.
2. Description of Related Art
A so-called resonant bidirectional DC/DC converter that utilizes the resonance phenomenon of an LC resonant circuit configured of a reactor and capacitor is disclosed in patent application publication JP-A-2012-70491 (e.g., paragraphs [0019] to [0029], FIG. 1, FIG. 7, FIG. 8) and in “Efficiency Improvement of AC/DC Power Station”, Panasonic Technical Report, Vol. 59, No. 3, pp. 4-11. Also, it is described in these documents that, in order to reduce loss and noise in drive circuits (bridge circuits) connected to the primary side and secondary side of an isolating transformer, thereby optimizing resonance operation, resonance frequency is caused to coincide in bidirectional power flow via the isolating transformer by adding a regulator circuit, or the like, and regulating the constants of the LC resonant circuit.
FIG. 5 is a circuit diagram showing an existing resonant bidirectional DC/DC converter. The circuit is an example wherein full bridge circuits are disposed with bilateral symmetry centered on an isolating transformer, and an LC resonant circuit is connected between the isolating transformer and each full bridge circuit as means of optimizing the bidirectional resonance operation, wherein insulated-gate bipolar transistors (IGBTs) to which free wheeling diodes are connected in anti-parallel are used as semiconductor switch elements configuring the full bridge circuits.
In FIG. 5, the circuit includes direct current voltage supplies 11 and 21 (the voltages thereof are taken to be V1 and V2 respectively), smoothing capacitors 12 and 22, bridge circuits 13 and 23 that operate as switching circuits or rectifier circuits, LC resonant circuits (series resonant circuits) 14 and 24, an isolating transformer 30, a primary coil 31 thereof, and a secondary coil 32. Also included are semiconductor switch elements Q1 through Q4 and Q5 through Q8 configuring the bridge circuits 13 and 23 respectively, resonant reactors 14a and 24a, and resonant capacitors 14b and 24b. When using this bidirectional converter as, for example, a battery charger, one of the direct current voltage supplies 11 and 21 forms the battery, while the other functions as a direct current power supply. G1 through G8 indicate gate signals (gate electrodes) of the semiconductor switch elements Q1 through Q8, and N1 and N2 indicate the numbers of turns of the coils 31 and 32, respectively.
In the heretofore described configuration, the bridge circuit 13 converts direct current power into alternating current power using switching operations of the semiconductor switch elements Q1 through Q4 when power flows from the direct current voltage supply 11 to the direct current voltage supply 21, and converts alternating current power into direct current power using rectifying operations of the free wheeling diodes when power flows from the direct current voltage supply 21 to the direct current voltage supply 11. In the same way, the bridge circuit 23 converts alternating current power into direct current power using rectifying operations of the free wheeling diodes when power flows from the direct current voltage supply 11 to the direct current voltage supply 21, and converts direct current power into alternating current power using operations of the semiconductor switch elements Q5 through Q8 when power flows from the direct current voltage supply 21 to the direct current voltage supply 11.
Herein, when power flows from the direct current voltage supply 11 to the direct current voltage supply 21, the voltage applied when there is reverse recovery of the free wheeling diodes of the semiconductor switch elements Q5 through Q8 is clamped to the voltage V2 of the direct current voltage supply 21. Also, when power flows from the direct current voltage supply 21 to the direct current voltage supply 11, the voltage applied when there is reverse recovery of the free wheeling diodes of the semiconductor switch elements Q1 through Q4 is clamped to the voltage V1 of the direct current voltage supply 11. According to this bidirectional DC/DC converter, low breakdown voltage elements that generally have low loss occurrence can be used as the semiconductor switch elements Q1 through Q8, and high conversion efficiency can thus be obtained.
It is known that, when arranging so that output voltage is variable in this kind of bidirectional DC/DC converter, the semiconductor switch elements Q1 through Q8 are driven using pulse frequency modulation (PFM) control, as disclosed in, for example, patent application publication JP-A-2011-120370 (e.g., paragraphs [0022] to [0044], FIG. 2). As is commonly known, PFM control is a control method whereby the duty ratios of the drive signals of the semiconductor switch elements Q1 through Q8 are changed by changing the switching frequency.
FIGS. 6A and 6B are configuration diagrams of control means for carrying out PFM control of the semiconductor switch elements Q1 through Q8. FIG. 6A is control means of the semiconductor switch elements Q1 through Q4 of the bridge circuit 13, and is configured of a detector circuit 42, which detects the voltage V2 and a current I2 of the direct current voltage supply 21, and a control circuit 51, which generates the gate signals G1 through G4 for carrying out PFM control of the semiconductor switch elements Q1 through Q4 based on values detected by the detector circuit 42. Also, FIG. 6B is control means of the semiconductor switch elements Q5 through Q8 of the bridge circuit 23, and is configured of a detector circuit 41, which detects the voltage V1 and a current I1 of the direct current voltage supply 11, and a control circuit 52, which generates the gate signals G5 through G8 for carrying out PFM control of the semiconductor switch elements Q5 through Q8 based on values detected by the detector circuit 41.
The semiconductor switch elements Q1 through Q4 are driven via a gate drive circuit (not shown) by the gate signals G1 through G4 output from the control circuit 51 when power flows from the direct current voltage supply 11 to the direct current voltage supply 21. Therefore, control whereby the voltage V2 of the direct current voltage supply 21 coincides with a command value is carried out. Also, the semiconductor switch elements Q5 through Q8 are driven via a gate drive circuit by the gate signals G5 through G8 output from the control circuit 52 when power flows from the direct current voltage supply 21 to the direct current voltage supply 11. Therefore, control whereby the voltage V1 of the direct current voltage supply 11 coincides with a command value is carried out. By carrying out PFM control of the semiconductor switch elements Q1 through Q8 using the control means shown in FIGS. 6A and 6B, output voltage can be variably controlled in bidirectional power flow.
Meanwhile, according to JP-A-2002-262569 (paragraphs [0002], [0003], and the like) (U.S. Pat. No. 4,951,185), the output voltage characteristics with respect to switching frequency change depending on the size of the load when PFM control is applied to a resonant DC/DC converter, and in particular, when there is a light load or no load, the output voltage cannot be controlled to or below a certain value even when the switching frequency is increased, and it is pointed out that application to an application wherein the input/output voltage range is wide, as with a battery charger, is difficult.